1. Field of the Invention
The present invention relates to a cable with recyclable covering layer. In particular, the invention relates to a cable for transporting or distributing medium or high voltage electric energy, wherein an extruded covering layer based on a thermoplastic polymer material in admixture with a dielectric liquid with good mechanical and electrical properties is present, enabling, in particular, the use of high operating temperatures and the transportation of high power energy.
Said cable may be used for both direct current (DC) or alternating current (AC) transmission or distribution.
2. Description of the Related Art
The requirement for products of high environmental compatibility, composed of materials which, in addition to not being harmful to the environment during production or utilization, may be easily recycled at the end of their life, is now fully accepted in the field of electrical and telecommunications cables.
However the use of materials compatible with the environment is conditioned by the need to limit costs while, for the more common uses, providing a performance equal to or better than that of conventional materials.
In the case of cables for transporting medium and high voltage energy, the various coverings surrounding the conductor commonly consist of polyolefin-based crosslinked polymer, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also crosslinked. The crosslinking, effected after the step of extrusion of the polymeric material onto the conductor, gives the material satisfactory mechanical and electrical properties even under high temperatures both during continuous use and with current overload.
It is well known however that crosslinked materials cannot be recycled, so that manufacturing wastes and the covering material of cables which have reached the end of their life may be disposed of only by incineration.
Electric cables are also known having their insulation consisting of a multi-layer wrapping of a paper or paper/polypropylene laminate impregnated with a large quantity of a dielectric liquid (commonly known as mass impregnated cables or also oil-filled cables). By completely filling the spaces present in the multi-layer wrapping, the dielectric liquid prevents partial discharges arising with consequent break down of the electrical insulation. As dielectric liquids products are commonly used such as mineral oils, polybutenes, alkylbenzenes and the like (see, for example, U.S. Pat. Nos. 4,543,207, 4,621,302, EP 987,718, WO 98/32137).
It is however well known that mass impregnated cables have numerous drawbacks compared with extruded insulation cables, so that their use is currently restricted to specific fields of application, in particular to the construction of high and very high voltage direct current transmission lines, both for terrestrial and in particular for underwater installations. In this respect, the production of mass impregnated cables is particularly complex and costly, both for the high cost of the laminates and for the difficulties encountered during the steps of wrapping the laminate and then of impregnating it with the dielectric liquid. In particular, the dielectric liquid used must have low viscosity under low temperatures to allow rapid and uniform impregnation, while at the same time it must have a low tendency to migrate during installation and operation of the cable to prevent liquid loss from the cable ends or from accidentally breaks on the cable. In addition, mass impregnated cables cannot be recycled and their use is limited to an operating temperature of less than 90° C.
Within non-crosslinked polymeric materials, it is known to use high density polyethylene (HDPE) for covering high voltage cables. HDPE has however the drawback of a lower temperature resistance than XLPE, both to current overload and during operation.
Thermoplastic low density polyethylene (LDPE) insulating coverings are also used in medium and high voltage cables: again in this case, these coverings are limited by a too low operating temperature (about 70° C.).
International Patent Application WO 99/13477 discloses an insulating material consisting of a thermoplastic polymer forming a continuous phase which incorporates a liquid or easily meltable dielectric forming a mobile interpenetrating phase within the solid polymer structure. The weight ratio of thermoplastic polymer to dielectric is between 95:5 and 25:75. The insulating material may be produced by mixing the two components while hot either batchwise or continuously (for example, by means of an extruder). The resultant mixture is then granulated and used as insulating material for producing a high voltage electric cable by extrusion onto a conductor. The material may be used either in thermoplastic or crosslinked form. As thermoplastic polymers are indicated: polyolefins, polyacetates, cellulose polymers, polyesters, polyketones, polyacrylates, polyamides and polyamines. The use of polymers of low crystallinity is particularly suggested. The dielectric is preferably a synthetic or mineral oil of low or high viscosity, in particular a polyisobutene, naphthene, polyaromatic, α-olefin or silicone oil.
International Patent Application WO 02/03398 in the name of the Applicant, discloses a cable comprising at least one electrical conductor and at least one extruded covering layer based on thermoplastic polymer material in admixture with a dielectric liquid, wherein said thermoplastic material comprises a propylene homopolymer or a copolymer of propylene with at least one olefin comonomer selected from ethylene and an α-olefin other than propylene, said homopolymer or copolymer having a melting point greater than or equal to 140° C. and a melting enthalpy of from 30 J/g to 100 J/g. Said dielectric liquid comprises at least one alkylaryl hydrocarbon having at least two non-condensed aromatic rings and a ratio of number of aryl carbon atoms to total number of carbon atoms greater than or equal to 0.6, preferably greater than or equal to 0.7.
International Patent Application WO 02/27731 in the name of the Applicant, discloses a cable comprising at least one electrical conductor and at least one extruded covering layer based on thermoplastic polymer material in admixture with a dielectric liquid, wherein said thermoplastic material comprises a propylene homopolymer or a copolymer of propylene with at least one olefin comonomer selected from ethylene and an α-olefin other than propylene, said homopolymer or copolymer having a melting point greater than or equal to 140° C. and a melting enthalpy of from 30 J/g to 100 J/g. Said dielectric liquid comprises at least one diphenyl ether, non-substituted or substituted with at least one linear or branched, aliphatic, aromatic or mixed aliphatic and aromatic C1–C30 hydrocarbon radical.
However, the prior art above cited presents some drawbacks.
As a matter of fact, Applicant noticed that the addition of a dielectric liquid to a polymer material should both determine a significant increase in its electrical properties (in particular, its dielectric strength), without impairing its thermomechanical characteristics and without resulting in exudation of the dielectric liquid from the polymer material. In particular, the resultant cable should give substantially constant mechanical and electrical performances with time and hence high reliability, even at high operating temperatures (at least 90° C. and beyond, in particular at operating temperature up to 110° C. for continuous use and up to 140° C. in the case of current overload). In particular, Applicant noticed that the presence of two phases, e.g. a continuous phase of a thermoplastic material and an additional phase incorporated therein of a dielectric liquid, with the consequent microscopically non homogeneous dispersion of said dielectric liquid onto said thermoplastic material, does not allow to obtain all the above reported characteristics.